I. Field of the Invention
This invention relates to a sheet metal forming tool and to a method of forming sheet metal.
II. Discussion of the Prior Art
The invention may be applicable to forming of containers or of articles of other shapes, e.g., motor vehicle panels. The invention is applicable to any material capable of being cold formed, such as steels and aluminum alloys.
Difficulties are met in forming hollow articles which do not have a constant cross-section, or have angled sides, due to the formation of wrinkles in metal being formed by a punch. This problem is particularly acute in drawing and forming thin steel.
One example where this problem arises is in the drawing and forming of a metal tray having tapered sides. The problem of wrinkling in this field was considered in GB-A-1349059 in relation to aluminum sheet. As explained in that document, one known method of trying to remove wrinkles when producing a hollow article with tapered walls is to use a tapered punch and to provide a resiliently flexible ring in the die. This is unsatisfactory for use with very thin metal, as is now produced by deep drawing techniques.
GB-A-1349059 discloses a more effective arrangement, in which the head of the punch mounts a ring of elastically deformable material. The ring closely fits in the mount of a hollow blank held in the die, but deforms to the tapered shape of the die. This arrangement is much more effective in removal of wrinkles.
One problem with this arrangement is that the deformable ring has to be under compression and the compressive force has to be accurately controlled. Too much resilience or too little resilience will result in a poorly formed article. GB-A-1349059 discloses several ways of providing compression.
In practice, it is difficult with this arrangement to vary and control the compression around the tool for effective production of non-circular articles.
It would be advantageous both in providing for simple manufacture of a punch, as well as to avoid the need for accurate setting of the compressive force, to avoid the necessity for having to control compression.
GB-A-2145962 also considers the problem of wrinkling and also uses an elastic ring which has a specifically controlled shape and a cavity between the ring and a solid core to take up deformed material from the elastic ring. Very careful manufacture and assembly is required for working of this arrangement.
GB-A-2145962 is concerned with manufacture of circular section trays. The problem of wrinkling is much less marked than with manufacture of hollow articles of non-circular section. The provision of a cavity between the ring and the core will not be able to solve the wrinkling problem in hollow articles of rectangular or more complex shape.
The present invention avoids the disadvantages of GB-A-13490569 and GB-A-2145962 and permits manufacture of non-circular articles with improved smoothness and lack of wrinkles, even in corners of the article.
The invention overcomes the wrinkling problem in the cold forming of articles of rectangular section or of irregular sections, as required, for example, in the motor vehicle industry. The invention also permits the cold forming of a hollow article, whose side wall has circumferential steps.